Intrathecal Administration of Iodinated Contrast Above the Labeled 300 mg/mL Ceiling: A 14-Year Retrospective Series of CT Myelograms

Background. Intrathecal iodinated contrast is labeled to a maximum of 300 mg/mL for both approved agents, iopamidol and iohexol, on the basis of seizure risk. That ceiling does not mark a demonstrated toxic threshold: published series show no concentration-response relationship, with seizures reported at low as well as high concentrations, and osmolality-based explanations have failed in animal and human testing. Higher-concentration formulations are more readily available than dedicated low-concentration preparations.

Methods. We retrospectively reviewed 74 consecutive CT myelograms performed between January 2012 and June 2026 using contrast above the labeled ceiling. Demographics, examination region, indication, agent, CNS-active medication use, and post-procedural events were extracted.

Results. Mean age was 58.7 ± 15.0 years (median 62, IQR 51–69; range 17–87); 54 examinations (73.0%) were in men. Whole spine accounted for 29 (39.2%), lumbar only 20 (27.0%), cervical only 13 (17.6%), the remainder thoracic or combined. Iopamidol 370 mg/mL was used in 51 (68.9%) and iohexol 350 mg/mL in 23 (31.1%). Spinal stenosis was the indication in 52 (70.3%); MRI was not feasible or incomplete in 13 (17.6%). Nineteen patients (25.7%) were taking a CNS-active drug: antiseizure in 6 (8.1%), psychiatric in 16 (21.6%), both in 3 (4.1%); antidepressants were the dominant class (12, 16.2%). No seizure occurred (0 of 74). Eight patients returned to the emergency department: five for headache, two for back pain, one for dizziness.

Conclusion. To our knowledge, this is the first series reporting intrathecal contrast above the labeled 300 mg/mL ceiling. No seizures or severe adverse events were observed. The limit is not supported by concentration-specific data, and requiring departments to stock a dedicated low-concentration agent for a rarely performed examination should rest on firmer evidence. The sample size precludes a formal safety claim.